Electric motors are nowadays employed for a plethora of different applications in essentially all fields of technology. Depending on the actual use of the respective electric motor, various types and various sizes of electric motors are employed.
As an example, if an electric motor has to be used for an application where a constant turning speed of the electric motor can be used or is even required, a synchronous electric motor without a commutator can be used, in particular if alternating current is available. However, a start-up of such an electric motor might be difficult if a load with a large moment of inertia has to be driven.
If, however, a variable turning speed of the electric motor has to be provided (and additionally in the case of a direct current source), the traditional approach was to use electric motors, comprising a commutator (so called asynchronous electric motors). A problem involved with such commutated electric machines is the commutator, since this is a component that is particularly subject to a non-negligible wear. Furthermore, when using a commutator, typically sparks occur when the electric motor is turning. Such sparks can render the resulting electric motor unusable for certain applications, in particular if flammable gases are around, unless additional precautions are taken.
With the advent of modern semiconductor-based power electronics, the use of synchronous electric motors, in particular synchronous reluctance electric motors, has become more and more widespread. With such synchronous electric motors, commutators can be dispensed with. Furthermore, by providing an alternating electric current with a variable frequency, a synchronous electric motor can be driven at essentially any rotating speed. Even changes in turning speed can be realised. This has made possible certain applications that were hard to achieve, if at all, beforehand.
With the increasing number of electric motors there is an increasing interest for simple methods for controlling electric motors. This is because this way the necessary electronics for controlling the electric motors can become simpler and hence smaller, less energy consuming, more reliable and in particular less costly.
Although a number of different controllers for electric motors and a variety of methods for driving electric motors are known in the state of the art, there is still a desire for further improvements.